I was looking through code like this:
foo = '%s%s%s' % ('https://', host, uri)
and realized this could be rewritten by the interpreter as:
foo = 'https://%s%s' % (host, uri)
I tried to determine how much code this might affect, but it was pretty hard for me to come up with a decent regex to filter out all the false positives. There were too many hits to determine if this would be used often.

> and realized this could be rewritten by the interpreter as:
Yeah, it could but it's tricky to implement it. The current peephole is implemented in C. You may first try to implement it using my astoptimizer project which is implemented in Python. At least to check if it's possible or not :-)
https://bitbucket.org/haypo/astoptimizer
astoptimizer only optimizes str%args if it succeed at compile time, so if all arguments are constant and no error is raised.